First they got rid of customizing message app. I loved picking a picture from my gallery as a background = GONE
I loved changing the chat bubbles and fonts in the message app = GONE
PhotoEditor Pro entigrated with normal editor, no censoring parts of a picture option
Samsung official stance is you can't rollback firmware once it's been installed.
However take a look on xdadevelopers site and Google Search and you'll find further information.
A person doing this does so at their own sole responsibility knowing that a phone can be put Into an unusable state and possible manufacturing warranty being voided.
Other customisation options could be accessed via using a 3rd party launcher such as Nova Prime etc.
Other Text messaging options such as Textra as one example may offer further customisation options.
Feedback to Samsung is best sent via the Samsung Members App / Samsung + App to which I've sent my own feedback on features I miss personally.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.